In general, a lighting device using light-emitting diodes (LEDs) comprises a base with LEDs mounted thereon, and a globe covering the LEDs. Light emitted from the LEDs is diffused to the environment through the globe.
In lighting devices using LEDs, there is a demand for realization of a light distribution angle, a total luminous flux, a size, and an appearance during lighting equivalent to those of a lighting device using a common light bulb, a fluorescent light, etc. There is another demand for installing, in a lighting device, additional functions, such as communication and storage, as well as a power supply circuit, such as an AC-DC conversion circuit.
Generally, in lighting devices using LEDs, in order to control the light distribution angle, it is necessary to design the shape and/or the diffusion coefficient of a globe so that the light forwardly emitted from the light emitting surface of the globe will be guided in a desired direction.
Furthermore, in order to increase the total luminous flux, it is necessary, in making the LED emit light, to increase the current. This inevitably increases the amount of heat emitted from the LED or a power supply circuit. Further, if additional functions, such as communication and storage, are used, the components associated therewith produce heat.
To prevent performance degradation of accessories including the power supply circuit, and the LEDs, it is necessary to reduce the thermal resistance of the lighting device and to suppress the temperatures of the LEDs and the accessories less than their respective allowable temperature limits. If the thermal resistance of each part of the lighting device is sufficiently low, the lighting device exhibits substantially uniform temperature, and the thermal resistance between the LEDs and the environment will be substantially minimum.
In this case, since the difference in temperature between the LEDs and the accessories is small, the allowable temperature limit of the lighting device is substantially equal to that of a component that has the lowest allowable temperature limit among the LEDs and the accessories. This makes it difficult to supply high electric power to the LEDs.
Thus, when the allowable temperature limits of the above-mentioned accessories are lower than that of the LEDs, it is difficult to realize high-output LEDs.
It is an object of the invention to provide a lighting device in which sufficient heat dissipation performance of a light source is maintained, while degradation of performance due to the production of heat by the light source and accessories is avoided.